subroutine teten(tk, es)
! This routine calculates the saturation vapor pressure using
! Teten's formula
!      input:  tk = temperature (K)
!      output: es = saturation vapor pressure (Pa)

! Modifications:
! 4/20/21 (J. Dostalek) Started this Fortran 90 version
!-------------------------------------------------------------------------------

  implicit none

  real, intent(in) :: tk
  real, intent(out) :: es

  real, parameter :: c1 = 610.78
  real, parameter :: c2 = 17.269
  real, parameter :: c3 = 35.86

! Check for valid temperature
  if (tk < 40.0) then
    es = 0.0
    return
  end if

  es = c1*exp(c2*(tk - 273.16)/(tk - c3))

  return
end subroutine teten